That's macroblocking and probably has to do with how YouTube converts the video, assuming it's not in your source video.

You might need to use encoding settings and a container more compatible with YouTube, possibly at an even higher bitrate.

One thing I have noticed is that if you upload a video over 1080p in resolution, but lower than 1440p, it will upscale to 1440p and be given a higher bitrate by YouTube that seems less prone to macroblocking.